Why did Lincoln address the issue of slavery during the war?

Lincoln "addressed" the issue of slavery before the war, in the sense that he spoke against it. However, he didn't actually do much about it until late in the war, when he issued the Emancipation Proclamation.

Lincoln is known to have said in a letter that he cared more about preserving the Union than he did about slavery, and whatever kept the United States together ... freeing the slaves, leaving them alone, or freeing some and leaving others alone ... he was willing to do.

We don't know his exact motivations, but we do know that one effect of the Emancipation Proclamation was to quell European support for the Confederacy ... it turned the US Civil War from a purely political matter (in which a divided US was to their advantage) to a moral issue (whether one supported slavery or not) in their eyes, Since the main powers of the time ... Britain and France ... had already abolished slavery, it made supporting the Confederacy politically difficult for them.